The present invention relates to a novel screw and tool in particular for use with a orthopedic implant system wherein the screw has a torque driving recess with a particular configuration designed for its intended use. The tool has a corresponding male shape, which in conjugation with the screw recess, is designed to achieve a controlled torque load with a minimized possibility of stripping the recess or of shearing the torque-driving tip from the tool. The configuration of the recess is constant along its length and in cross section appears as a central circle with multiple intersecting circles on its circumference.
Orthopedic implant systems, such as those used in the spine, are finely engineered constructs which are designed to react to certain loads applied by the muscular-skeletal system of the patient. These systems may bear weight, if for example, the skeleton is unable to sustain the load. The system may be used to counteract loads such as for deformities in alignment. Since these systems are implanted into the body, it is critical that they are designed to accomplish the intended tasks with low profiles and minimal geometries; with smooth and minimally invasive tissue contacting surfaces; and with an ease of assembly that facilitates surgical implantation.
A spinal implant system typically consists of one or two stabilizers, such as a rod, plate, or occasionally, a cable. These are held in place relative to the bone by a fastener, which is usually a screw or a hook. In particular for rods, the fasteners usually hold an anchor member with a channel or bore for the rod. In the case of a channel, means are provided to close the rod in the channel, such as a threaded lock which screws onto, or into threads on the anchor, typically exterior to the rod relative to the bone. These means can also include a closure member which is slid, or moved longitudinally into place on the anchor recess. Such a closure member may include mating surfaces such as flanges in the form of dovetails or cut back recesses that assure that the closure member will have a limited degree of freedom relative to the anchor member during surgical assembly. Of course, other locking or closure means which may be used are also contemplated with respect to the screw of the present invention.
It is often critical to the proper functioning of these systems that the stabilizer can be secured in position in the anchor. This often means restraining the stabilizer from slipping longitudinally. This may be particularly difficult when the system is used to manipulate the skeletal system, such as for a curvature of the spine. Often, very small fastening means such as screws or nuts are used in the anchor to retain the stabilizer in place. These screws, which may be referred to as set screws or fastening screws, are received in threaded recesses in the anchor, or closure member, and tightened onto the stabilizer to hold it in place. Alternatively, an anchor member may include a nut which closes a rod recess and secures the rod in place. Collectively, these fastening means as well as other possible clamping elements are referred to herein as fasteners.
With the surgical environment, it is critical that the implant system function reliably, and goes together easily in spite of the very small size. As for any surgery, time is of the essence for surgical implantation. For spinal surgery, many components may have to be assembled so that a time savings for each anchor assembly is multiplied by anywhere from two to up to twenty when a pair of rods is used to span multiple vertebrae. Moreover, the assemblies are designed to avoid mechanical complications, such as cross-threading, stripped threads, unintentional shearing of parts, splintering, fussy assembly, and components or metallic parts which are hard to handle and could drop into or contaminate the surgical site.
Since the fasteners of the prior art assemblies have often been required to generate a relatively large amount of torque, i.e. from 50 to 100 or more inch-pounds, they are typically designed with internal hexagon torque recesses. However, a problem with these fasteners is that the hex can be stripped so that the screws cannot be tightened, or backed out without a special tool. The prior art has recognized that this problem exists, and special shapes including for example, the sinusoidal torx recess, have been devised in order to achieve high torque for small internal recesses. However, the problem with this shape is that the recess strips out fairly easily or tip of the driving tool can be sheared off. While for many applications, this would be inconvenient. It is especially important for an implant assembly where the tip could fall into the surgical site and where the assembly and disassembly must be smooth.
Of course, this screw and torque driving tool has other applications which can utilize the advantages of this invention.
The present invention provides a fastener, specifically including a screw or nut, having a unique torque recess which is a central cylindrical recess having equi-spaced cylindrical grooves set about the circumference of the central recess. In cross-section, this shape appears to be a circle having 2 to 6 equally spaced lobes, which are preferably also circular. The lobes do not overlap each other. They do overlap the circumference of the central circle. Consequently, there are areas between each of the hemicircular lobes which are an arc of the central circle. Alternatively, these joining sections can also be flat. It is however preferable that the lobes form a sharp distinction, or crease, where they join the central recess, and that they have a curve shaped away from the central recess which is preferably circular, but could also be oval, or some complex shape. The recess has a constant shape along its longitudinal length, i.e. its depth.
There is a top concave opening to the recess which forms the transition between a planar top surface and the recess. The screw has an outer circumference which preferably include flats on the edge in order to better enable it to be started by finger tightening, if desired. The outer circumference has from about 4 to about 8, and preferably 6 flats separated by small rounded joints. Further, the area between the side and the flat top is chamfered to avoid sharp edges.
The tip of the tool has a mating correspondence to the recess. There is a cylindrical central core which is bounded by a equally spaced series of cylinders which intersect the circumference of the central core, such as at the center of the lobes. Accordingly, the areas of the tip which separate the outer cylinders appear as grooves or creases in the external surface of the tip. The bottom of the each groove is convex as it is defined by the arc of the central core, although it is contemplated that this area can also be flat. As has been mentioned for the recess, a sharp delineation forms where each of the outer cylinders join the central core. The tip has a transition area where the diameter decreases from a larger cylindrical shape to an area of triangular lands separated by the curving grooves. A triangular land joins and intersects an outer cylindrical groove. The depth of the constant cross-section area is substantially the same as the depth of the recess. The bottom is flat, but could have other configurations. There are from 2 to 6 peripheral cylinder portions (i.e. lobes), but more preferably, there are 4 or 5 with five being most preferred. In a second embodiment, the recess includes interior threads with a minor diameter that corresponds to that of the central core, and a major diameter roughly corresponding to the diameter of a circle which is the lobe to lobe measurement for lobes which are opposing on the core. This results in a series of flights of flanges in the recess that will accept an internal screw.
Thus it is an object of the present invention to provide a screw having a torque driving recess that will allow generation a torque of about 100 to about 150 in-lb. In some cases the goal will be a torque of about 20-50 in-lb. It is an object to provide a screw and corresponding tool for an orthopedic implant system which reduces the risk of internal stripping of the torque driving recess and balances it with a reduction of the risk of shearing the tip of the torque driving tool.